The introductionary sections of as-module and cli-options have been
rewritten. Further, Instaloader's short-description is now included in
the sidebar.
caption_hashtags is a list of all hashtags that are mentioned in the
Post's caption. It allows to easily filter Posts that have multiple
hashtags, and as such fixes #24.
Further, the documentation of --only-if has been completed by linking to
a description of the syntax in the Python documentation, and by linking
to a list of all defined properties with their meanings. So, this commit
also closes #42.
